As is well known, the power and efficiency of gas turbine engines typically increases with increasing nominal operating temperature, but the ability of the turbine to operate at increasingly higher temperatures is limited by the ability of the turbine components, especially the vanes and blades, to withstand the heat, oxidation and corrosion effects of the impinging hot gas stream and still maintain sufficient mechanical strength. Thus, there exists a continuing need to find advanced material systems for use in components that will function satisfactorily in high performance gas turbines, which operate at higher temperatures and stresses.
One approach to providing improved turbine components is to fabricate a strong, stable substrate having the shape of the component, and cover the substrate with a thin protective coating that resists the oxidation and corrosion effects of the hot combustion gas stream. The underlying substrates, usually nickel-base or cobalt-base superalloy compositions, were at one time formed by common forging or simple casting procedures but now improved performance results from use of cooled airfoils made by directional solidification or directional recrystallization procedures. Even greater operating temperatures are possible by casting the substrate as a single crystal having no grain boundaries which might cause premature failure, and with the single crystal orientation selected to meet required creep-rupture and fatigue lives.
Insulative ceramic coatings further enhance turbine performance by reducing heat transferred into cooled airfoils, reducing the requirement for cooling air, which is a performance penalty. Durability of turbine components is also enhanced by ceramic coatings that minimize metal temperatures and thermal stresses in the superalloy component.
A modern ceramic coating system typically has several layers of differing compositions, and properties, in order to provide the best combination of benefits. For example, one layer may be relatively thick and porous to provide an insulative effect but, by itself, offering little resistance to oxidation, erosion, or corrosion. The outer surface of such a layer may be protected from erosion by providing a thin, hard, dense surface layer.
Generally, a thin metallic layer or bond coating is applied under the ceramic to protect the substrate through formation of an adherent oxide scale, such as aluminum oxide, which resists the oxidizing effects of the hot combustion gas stream. Other elements present in the coating contribute to the ability of the protective ceramic coating to adhere to the substrate through many cycles of gas turbine startup and shut down.
Lives of ceramic coatings are limited at high temperatures due to excessive growth of the oxide scale on the bond coating and flaws which develop within the interfacial zone between the metallic bond coating and insulative ceramic layer. Thermally induced deterioration of the interfacial zone coupled with thermal, and ceramic-superalloy thermal expansion mismatch, stresses eventually lead to spalling of the insulative layer.
It should be apparent from the foregoing general discussion of the art, that further improvements, in both the effectiveness and useful life, of coating systems are required in order to survive the increasingly severe operating conditions in high performance gas turbine engines.
It is therefore an object of the present invention to provide a new and improved ceramic based coating system for use on gas turbine engine components.
Another object of this invention is to increase the cyclic oxidation life of ceramic coatings on gas turbine airfoils.
A further object of this invention is to provide improved methods of applying ceramic coatings to metallic substrates.